OTDR is an established technique for analysing the propagation of light in an optical fibre. In the telecommunications industry, the technique is widely used to detect and locate damage to optical fibres. The amount of light Rayleigh backscattered in an optical fibre as a light pulse travels along the fibre can be detected using a photodetector arranged at the end of the optical fibre into which the light pulse is transmitted. Analysing a signal generated by the photodetector representative of the detected backscattered light over time can allow determination of a spatial distribution of the amount of light backscattered at different points along the fibre. As more light is either absorbed or backscattered at locations of damage or such like, these locations can be identified from the determined spatial distribution.
A related technique, known as phase sensitive OTDR, recognises that when the light pulse is coherent and propagates in a monomode optical fibre, components of the light Rayleigh backscattered from the light pulse interfere with each other to generate a so-called temporal speckle pattern at the photodetector. The intensity of the temporal speckle pattern at any given moment depends on phase differences between the different components of Rayleigh backscattered light arriving at the photodetector at that moment. These components have been backscattered from the light pulse when it was at a corresponding spatial location in the fibre. Consequently, the momentary intensity of the temporal speckle pattern depends on conditions affecting the phase of light across the spatial extent of the light pulse at that location, e.g. the local refractive index of the fibre. Any local variation in these conditions between successive light pulses will result in a difference between the momentary intensities of the temporal speckle patterns of the respective light pulses corresponding to that location. Comparing signals generated by the photodetector for coherent light pulses transmitted successively along the fibre can therefore allow changes in the local refractive index, e.g. caused by external influences such as strain exerted on the fibre, to be detected and located.
It is well known that, for conventional OTDR, the temporal speckle pattern generated by any coherent component of the light pulse constitutes noise in the desired photodetector signal, which, in the absence of damage or such like, should ideally represent a smooth spatial distribution of backscattered light. Generally, light pulses used for conventional OTDR are therefore fairly incoherent, having spectral widths greater than around 500 GHz. This reduces the relative contribution of the temporal speckle pattern to the intensity of the backscattered light received at the photodetector.
On the other hand, for phase sensitive OTDR, backscattered light from any incoherent component of the coherent light pulses does not contribute to the temporal speckle pattern and therefore reduces the level of the wanted signal in the desired comparison between photodetector signals for successive light pulses. The comparison or “difference signal” should ideally just represent changes in the conditions affecting phase across the spatial extent of the light pulses at different locations along the fibre caused by changes in external influences occurring between transmission of the respective light pulses. The presence of differences due to changes in backscattering of incoherent light is undesirable.
So, in an implementation of phase sensitive OTDR described in U.S. Pat. No. 5,194,847 and improved by the paper “Polarisation Discrimination in a Phase-Sensitive Optical Time-Domain Reflectometer Intrusion-Sensor System”, Juan C. Juarez et al, Optics Letters, Vol. 30, No. 24, 15 Dec. 2005, it is stipulated that the light pulses should be very coherent. More specifically, U.S. Pat. No. 5,194,847 states that the spectral width of the light source should be of the order of 10 kHz and the more recent paper states that the spectral width of the light source should be less than 3 kHz. This narrow spectral width is intended to reduce the contribution to the photodetector signals of light backscattered from the incoherent component of each light pulse.
One problem with this implementation is that cheap sources of very coherent light are not readily available. Bespoke light sources have been designed to try to meet the coherence requirements of U.S. Pat. No. 5,194,847 and the above paper, e.g. as described in the paper “Spectrally Stable Er-Fibre Laser for Application in Phase Sensitive Optical Time-Domain Reflectometry”, Kyoo Nam Choi et al, IEEE Photonics Technology Letters, Vol. 15, No. 3, Mar. 2003, but these tend to be expensive. They are also prone to frequency drift. If frequency drift causes the frequency of the light source to change too much between successive light pulses, false differences between successive photodetector signals can be generated. Naturally, this limits the effectiveness of the technique.
Another problem is that the power of light pulses that can be launched into the optical fibre from coherent light sources is limited by various phenomena, particularly so-called “non-linear effects”. Notably, Brillouin scattering causes light to be inelastically backscattered (e.g. converted to backwardly propagating light of a different wavelength to that of the light pulse), resulting in attenuation of the light pulse as it travels along the optical fibre. Brillouin scattering occurs at all light pulse powers, but over a given power threshold it increases significantly. Crucially, this power threshold depends on the spectral width of the light pulse. For a light pulse having a spectral width less than around 17 MHz and wavelength around 1550 nm travelling in a 10 km long single mode optical fibre made of silica, the power threshold is around 5 mW. This therefore limits the power of the light pulses used in the implementation described in U.S. Pat. No. 5,194,847 and the above paper. However, it will be appreciated that there is a requirement for the photodetector to receive as much backscattered light as possible in order to generate a useful signal. So, the implementations described in U.S. Pat. No. 5,194,847 and the above paper try to meet this need by increasing the duration of the light pulses rather than the power of the light pulses. Indeed, in U.S. Pat. No. 5,194,847 the light pulses are described as being approximately 100 ns in duration and in the improved version of the technique described in the paper the light pulses are described as being 2 μs in duration.
It will be understood that the location of a disturbance in the propagation of light in the optical fibre can only be resolved to the spatial extent of the light pulse in the fibre, as the intensity of the temporal speckle pattern at any given moment constitutes the sum interference of the light backscattered from each spatial point in the light pulse at an associated moment. The spatial extent of the light pulses therefore defines the maximum possible spatial resolution of the technique. A light pulse of 2 μs duration has a spatial extent of around 200 m, meaning that the maximum possible spatial resolution of the technique described in the above paper is 200 m. This is far from ideal.
One way to improve spatial resolution without increasing the power of the light pulses is to use light pulses of shorter duration, but average photodetector signals generated by light backscattered from a number of the light pulses to generate a stronger signal and then to carry out the comparison using successive such averaged signals. However, this reduces temporal resolution. In other words, it takes longer to identify changes in the temporal speckle patterns and hence changes in external influences on the optical fibre. Furthermore, it becomes impossible to resolve changes that are faster than the duration of the averaging time. Applications such as detection of acoustic waves and so on cannot therefore be realised.
The present invention seeks to overcome these problems.